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Old 12th April 2010, 01:24 AM   #1
fury is offline fury  Australia
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Default JBL w15GTI sub

Hey,

I have a pair of these drivers and looking to create some boxes for them.
They will be paired up with JBL 2206h 12" midbass/midranges and BMS4550 in QSC waveguides.
Crossover will most likely be below 100hz.

The sub boxes will most likely be placed under the monitors (well big bookshelves), and be the stand for them.

Specs can be found here:
http://www.jbl.com/resources/Brands/...5GTi_rev_f.pdf


JBLs recommend sealed box rolls off a bit early (although size is great) f3 around 45hz.
JBL's ported box (4cu.ft tuned to 28hz) also models quite well, with an f3 of ~27hz. My only concern is that group delay starts to get high, around 20ms at 25hz.
(their graphs show it different to BBP which I use).

BBP recommends around 6.5cu.ft tuned to 20hz, which is getting too large for my room, but also group delay is increasing substantially to about 30ms at 20hz.

How audible is this group delay in the lower frequencies? How much is considered "acceptable"?

Should I build the 4cu.ft tuned to ~28hz box, or the biggest sealed box I can fit... or... any other suggestions?

Thanks!

Last edited by fury; 12th April 2010 at 01:27 AM.
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Old 12th April 2010, 09:13 AM   #2
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Have a read of these articles by Rod Elliot.
Personally, I believe vented enclosures produce audible group delay (well above 20ms), and so I only use sealed.
Vented boxes are also larger and harder to keep rigid.

Phase, Time and Distortion in Loudspeakers
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Old 12th April 2010, 11:34 PM   #3
fury is offline fury  Australia
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Interesting article... unfortunately doesn't state much about the audibility in subbass region, apart from and example where "The maximum delay is 55ms at around 18Hz. Although this may seem to be "bad", it is actually a very good result."

I guess the other alternative would be sealed box with a linkwitz transform (or eq of some sort) to equalize the low end roll off.

I will be using these with a DCX2496 so it's entirely possible.
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Old 13th April 2010, 03:41 AM   #4
Pallas is offline Pallas  Pakistan
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I had one in a 4.5 cubic foot box with EQ to bring up the low and, and it was excellent. Better than most drivers in "undersized" boxes, because it has such good thermal design.
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Old 20th April 2010, 02:03 AM   #5
NEO Dan is offline NEO Dan  United States
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What's going between the speakers?
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Old 20th April 2010, 09:39 AM   #6
fury is offline fury  Australia
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Speaker spikes most likely... probably a 1" space between top cabinet and bass cabinet.... I may even fill it with foam.
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Old 20th April 2010, 11:14 AM   #7
bjorno is offline bjorno  Sweden
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Quote:
Originally Posted by fury View Post
Interesting article... unfortunately doesn't state much about the audibility in subbass region, apart from and example where "The maximum delay is 55ms at around 18Hz. Although this may seem to be "bad", it is actually a very good result."

I guess the other alternative would be sealed box with a linkwitz transform (or eq of some sort) to equalize the low end roll off.

I will be using these with a DCX2496 so it's entirely possible.
Hi fury,

Agree, IMO, If calculated: Allowed gldy at 18 Hz = 1/18 = 55.6 mS for a good design but is in reality often totally masked by the room modals.

See a suggestion that doesn’t need LT to reach below 20 Hz:

b
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File Type: jpg JBL-W15GTi-T-TQWT-180L.jpg (192.4 KB, 311 views)
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Old 21st April 2010, 01:22 AM   #8
NEO Dan is offline NEO Dan  United States
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Quote:
Originally Posted by fury View Post
Speaker spikes most likely... probably a 1" space between top cabinet and bass cabinet.... I may even fill it with foam.
Hehe, I meant between the right and left speakers.
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Old 21st April 2010, 07:25 AM   #9
fury is offline fury  Australia
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Ahh, about 1.5 to 1.8mtrs in this room (yes quite close)...
2.5 to 3mtrs if i move them into the living room.


This is the current plan:
Click the image to open in full size.
Bass cab is approx 3cu.ft sealed...

The TQWT looks like it would be quite a large enclosure, probably too large for my room at this stage.
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Old 22nd April 2010, 05:23 PM   #10
bjorno is offline bjorno  Sweden
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Quote:
Originally Posted by fury View Post
Ahh, about 1.5 to 1.8mtrs in this room (yes quite close)...
2.5 to 3mtrs if i move them into the living room.

..Bass cab is approx 3cu.ft sealed...

The TQWT looks like it would be quite a large enclosure, probably too large for my room at this stage.
Hi fury,

Your suggested sub uses about 50 % more floor space than a T-TQWT does and (IMO) you need only one placed anywhere if XO: ed at 80 Hz or below.

Not forgetting the huge power losses if your closed sub boxes are LT: ed:
The power loss would be more than 20:1 if to compete with the T-TQWT low FR end.

I came to the conclusion that you would get a flatter upper bass if you use your JBL2206h in a tower stand using a volume giving a 70-80 Hz f-3dB point.

Then place your subs (preferably a T-TQWT and two scattered smaller ones with a f-3dB at least below 50 Hz) where the best overlapping mid bass room response is achieved.

If you move your system to the living room there is no doubt (IMO) that this approach would also resulting in an improved soundstage too.

b
Attached Images
File Type: gif 1-fury_LT.GIF (42.9 KB, 234 views)
File Type: gif 2-system-room-example.GIF (65.0 KB, 235 views)
File Type: gif 3-JBL-2206h-closed.GIF (57.9 KB, 236 views)
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